The announcement Tuesday by Congressman Castle (R) of Delaware that he’s running for Vice President Joe Biden’s old Senate seat potentially tees up one of the hottest races of the 2010 midterms.

With the Democrats controlling 60 seats in the Senate – giving them a filibuster-proof majority – they are loath to give the Republicans any quarter, especially with a seat the Democrats have held for decades. For the GOP, Castle is the only candidate who has a shot at taking that seat. If he joins the Senate, he would be a rare moderate Republican voice in an increasingly polarized body.

The vice president’s son Beau, Delaware’s elected attorney general, is expected to jump into the race as well. That would set up a generational battle – Beau Biden is 40, Castle is 70 – and a contest between two of the surest political brands in the state.

In his announcement press conference, held in Wilmington, Del., the mild-mannered Castle made clear he knows he’s in for a spirited contest if Biden runs.

“I like Joe and I like his family a great deal,” Castle said. “And that is a very political family, by the way, very competent in the art of politics, and I realize when you deal with any of the Bidens you’re dealing with all the Bidens.”

“I don’t know Beau as well, but obviously I respect him, too,” Castle added, noting Biden’s recent return from a year of service in Iraq.

Though Delaware is a sold blue state, party affiliation is only one of many factors voters will consider – especially given Castle’s moderate positions (such as support for abortion rights and embryonic stem cell research). Democrats, who control the White House and both houses of Congress, have been under gun, boosting GOP prospects for 2010.

Then there’s the Mike factor.

“Many voters don't seem to care about Mike Castle's positions on the issues,” says Allan Loudell, a talk radio host at WDEL-AM in Wilmington. “They just like the guy.”

The two Senate party committees reacted quickly to Castle’s announcement.

The Democrats came out first, a half hour before Castle’s expected announcement, and attempted to lash Castle to the legacy of President Bush.

“After four decades in politics, it is clear which direction Mike Castle wants to move Delaware and the country,” the Democratic Senatorial Campaign Committee said in a statement. “He built-up a record of supporting the George Bush’s economic policies, including tax cuts for the super-wealthy, that drove Delaware's economy into a ditch – and now won’t support any of the Obama-Biden plans to fix it. This will be a race about who is positioned to lead Delaware into the future, and Democrats fully intend to hold onto the Vice-President's seat.”

The Republicans followed with their opening salvo as Castle was speaking:

“Mike Castle’s announcement today instantly transforms Delaware into one of the most competitive Senate races in the country in 2010,” said Sen. John Cornyn (R) of Texas, chairman of the National Republican Senatorial Committee in a statement. “As an independent and experienced statesman who has earned the respect of Republicans and Democrats alike, Congressman Castle has a proven record of representing the people of Delaware in a bipartisan fashion, and we are thrilled that he has decided to take this next step toward extending his career of public service in the United States Senate.”